It is well known to use extruded tube structures of polytetrafluoroethylene (PTFE) as implantable intraluminal prostheses, particularly vascular grafts. PTFE is particularly suitable as an implantable prosthesis as it exhibits superior biocompatibility. PTFE tube structures may be used as vascular grafts in the replacement or repair of a blood vessel as PTFE exhibits low thrombogenicity. In vascular applications, the grafts are manufactured from expanded polytetrafluoroethylene (ePTFE) tube structures. These tube structures have a microporous micro-structure which allows natural tissue ingrowth and cell endothelization once implanted in the vascular system. This contributes to long term healing and patency of the graft. Grafts formed of ePTFE have a fibrous state which is defined by the interspaced nodes interconnected by elongated fibrils.
It is known to form an ePTFE tube structure from an extruded PTFE green tube structure. Such a green tube structure can be longitudinally expanded into an ePTFE tube structure which has a substantially larger longitudinal dimension in relation to the initial longitudinal dimension of the green tube structure. Such longitudinal expansion creates a node and fibril micro-structure which may be locked by sintering the ePTFE tube structure. The sintered ePTFE tube structure may then be further expanded to further change the dimensions or other characteristics or properties of the ePTFE tube structure. For example, the sintered ePTFE tube structure may be radially expanded to increase the diameter thereof.
One disadvantage of known methods of expanding an extruded PTFE green tube structure is that such methods typically provide for the green tube structure to initially be longitudinally expanded and then fully sintered. Further expansion of such a fully sintered ePTFE tube structure, such as by radial expansion thereof, may be difficult. For example, radial expansion of a fully sintered ePTFE tube structure typically requires very careful application of radial forces to the tube structure to prevent damage thereto during such expansion. More specifically, very precise force magnitudes and application rates thereof may be required, for example, to prevent tearing of the tube structure. Such precise control of the force application and possibly other conditions makes such further expansion difficult. The difficulty of such further radial expansion increases as the magnitude of such expansion increases. If further expansion of a fully sintered ePTFE tube structure is not properly performed, then significant portions of the sintered ePTFE tube structures may be damaged by such further expansion.
Thus, there is a need for a method for making PTFE tube structures for vascular grafts to facilitate further expansion of longitudinally expanded ePTFE tube structures.